The present invention relates to a pixel circuit disposed every pixel for current-driving a corresponding electroluminescence element and a method of driving the same. The present invention also relates to a display device having the pixel circuits disposed in matrix, especially, the so-called active matrix type display device for controlling an amount of current caused to flow through an electroluminescence element such as an organic EL element by using an insulated gate field-effect transistor provided within each pixel circuit, and a method of driving the same.
In an image display device, e.g., a liquid crystal display device, a large number of liquid crystal pixels are arranged in matrix. An image is displayed by controlling transmission intensity or reflection intensity of incident light every pixel in correspondence to information on an image to be displayed. While this is also applied to an organic EL display device having organic EL elements used in pixels, and the like, unlike the liquid crystal pixel, the organic EL element is self-light emitting element. For this reason, the organic EL display device has such advantages that it has higher visibility of an image than that in the liquid crystal display device, a back light is unnecessary, and a response speed is high. In addition, the organic EL display device is largely different from the liquid crystal display device which is of a voltage-controlled type in that it is of the so-called current-controlled type in which a luminance level (gradation) of each electroluminescence element can be controlled based on a value of a current caused to flow through the corresponding electroluminescence element.
In the organic EL display device, similarly to the liquid crystal display device, a simple matrix system and an active matrix system are known as a driving system thereof. Though the former is simple in construction, it involves such a problem that it is difficult to realize a large and high-definition display device, and so forth. Hence, at present, the organic EL display device using the active matrix system is actively being developed. This system is such that a current caused to flow through the electroluminescence element provided inside each pixel circuit is controlled by an active element (generally a thin film transistor (TFT)) provided inside the pixel circuit. The organic EL display device using this system is described in the following patent documents (Japanese Patent Laid-Open No. 2003-255856, Japanese Patent Laid-Open No. 2003-271095, Japanese Patent Laid-Open No. 2004-133240, Japanese Patent Laid-Open No. 2004-029791, Japanese Patent Laid-Open No. 2004-093682.)
FIG. 22 is a schematic block diagram showing a conventional organic EL display device using an active matrix system. As shown in the figure, this display device is constituted by a pixel array 1 as a main portion, and a peripheral circuit portion. The peripheral circuit portion includes a current driver 3, a light scanner 4, a drive scanner 5, and a scanner 7 for correction. The pixel array 1 is constituted by row-distributed lines WS, column-distributed signal lines SL, and pixels R, G and B which are disposed in matrix in places where the row-distributed lines WS and the column-distributed signal lines SL cross each other. While the pixels of the three primary colors of RGB are prepared in order to make the color display possible, single color pixels for black-and-white display are be used instead in some cases. The pixels R, G and B are constituted by pixel circuits 2, respectively. The signal line SL is driven by the current driver 3, so that a signal current is caused to flow through the signal line SL. The scanning lines WS are scanned by the light scanner 4. Incidentally, different scanning lines DS and AZ are also distributed in parallel with the scanning lines WS. The scanning lines DS are scanned by the drive scanner 5. The drive scanner 5 controls an electroluminescence period of an electroluminescence element included in each pixel. The scanning lines AZ are scanned by the scanner 7 for correction. The light scanner 4, the drive scanner 5 and the scanner 7 for correction constitute a scanner portion as a whole. The scanner portion successively scans the rows of the pixels every one horizontal period.
FIG. 23 is a circuit diagram showing an example of a structure of the pixel circuit shown in FIG. 22. As shown in the figure, the pixel circuit 2 is constituted by four transistors Tr1, Tr4, Tr5 and Trd, one pixel capacitor Cs, and one electroluminescence element EL. The four transistors are all thin film transistors. Of those transistors, the transistors Tr1, Tr4 and Tr5 are switching transistors for control, and are of an N-channel type each. On the other hand, the transistor Trd is a drive transistor for driving the electroluminescence element EL and is of a P-channel type. In addition, the electroluminescence element EL is a two-terminal type self-light emitting element including an anode and a cathode. For example, an organic EL element can be used as the electroluminescence element EL.
A source S of the drive transistor Trd is connected to a power source Vcc. A drain D of the drive transistor Trd is located on the anode side of the electroluminescence element EL. The cathode side of the electroluminescence element EL is grounded. A gate G of the drive transistor Trd is connected to one end of the pixel capacitor Cs. The other end of the pixel capacity Cs is connected to the power source Vcc.
A source/drain of the switching transistor Tr1 is connected between the signal line SL and the gate G of the drive transistor Trd. A gate of the switching transistor Tr1 is connected to the scanning line WS. A source/drain of the switching transistor Tr4 is connected between the gate G and drain D of the drive transistor Trd. A gate of the switching transistor Tr4 is connected to the scanning line AZ. A source/drain of the switching transistor Tr5 is connected between the drain D of the drive transistor Trd and the anode of the electroluminescence element EL. A gate of the switching transistor Tr5 is connected to the scanning line DS. The drive transistor Trd operates in a saturated region, and its characteristics are expressed by Expression 1:
      I    ds    =                    k        ⁢                                  ⁢        μ            2        ⁢                  (                              V            gs                    -                      V            th                          )            2      
In Expression 1, Vgs is a gate voltage and represents a voltage developed across the source S and gate G of the drive transistor Trd. Ids is a drain current and caused to flow through the source S and drain D of the drive transistor Trd to be supplied to the electroluminescence element EL. Vth represents a threshold voltage of the drive transistor Trd. μ represents carrier mobility of the drive transistor Trd. Also, k is a constant and given by Cox·W/L where Cox, W and L are a gate capacity, a channel width, and a channel length of the drive transistor Trd, respectively. The constant k is called a size factor in some cases. As apparent from Expression 1, when the drive transistor Trd operates in the saturated region, the drain current Ids starts to be caused to flow from a time point when the gate voltage Vgs exceeds the threshold voltage Vth. The magnitude of the drain current Ids increases in proportion to the square of the gate voltage Vgs. Incidentally, in this specification, it is assumed that the threshold voltage Vth of the drive transistor Trd takes its absolute value. By the way, since the threshold value of the P-channel transistor has a negative value, when this value is substituted into Expression 1 as it is, this is not proper. For this reason, in this specification, the threshold voltage takes its absolute value, and thus the threshold voltage Vth is treated as a positive value.
The drive transistor Trd, for example, is a TFT having an active layer made of a polycrystalline silicon thin film. Low-temperature polysilicon which is crystallized in the laser annealing process is used in the polycrystalline silicon thin film in many cases. In general, the low-temperature polysilicon TFT has a tendency to disperse in threshold voltage Vth and carrier mobility μ every device. In other words, the threshold voltage Vth and carrier mobility μ of the drive transistor Trd differ among the individual pixel circuits 2.
An operation of the pixel circuit 2 is roughly classified into a sampling operation and an electroluminescence operation. In the first sampling operation, the pixel circuit 2 turns off the switching transistor Tr5, while it turns on the switching transistors Tr1 and Tr4. When the current driver 3 drives the signal line SL in this state, a signal current Isig is caused to flow from the power source Vcc into the signal line SL through the drive transistor Trd, and the switching transistors Tr4 and Tr1. The operating characteristics of the drive transistor Trd at this time are expressed by Expression 2:
      I    sig    =                    k        ⁢                                  ⁢        μ            2        ⁢                  (                              V            gs                    -                      V            th                          )            2      
Expression 2 is expressed such that the drain current Ids in Expression 1 is replaced with the signal current Isig.
A gate voltage Vgs which is developed across the gate G and source S of the drive transistor Trd when the signal current Isig is caused to flow is expressed by Expression 3 by solving Expression 2 for
            V      gs        ⁢          V      gs        =                              2          ⁢                      I            sig                                    k          ⁢                                          ⁢          μ                      +                  V        th            .      
The gate voltage Vgs expressed by Expression 3 is held in the pixel capacitor Cs. In such a manner, in the sampling operation, the gate voltage Vgs corresponding to the level of the signal current Isig supplied by the current driver 3 is written to the pixel capacitor Cs. In brief, the signal current Isig is written to the gate of the drive transistor Trd.
Next, in the electroluminescence operation, the switching transistors Tr1 and Tr4 are turned off, while the switching transistor Tr5 is turned on. As a result, a drive current Ids is caused to flow from the drive transistor Trd into the electroluminescence element EL, so that the electroluminescence element EL emits light at predetermined luminance. The drive current Ids which is caused to flow through the drive transistor Trd at this time is expressed by Expression 4:
                              I          ds                =                                            k              ⁢                                                          ⁢              μ                        2                    ⁢                                    (                                                V                  gs                                -                                  V                  th                                            )                        2                                                  =                                            k              ⁢                                                          ⁢              μ                        2                    ⁢                                    (                                                                                          2                      ⁢                                              I                        sig                                                                                    k                      ⁢                                                                                          ⁢                      μ                                                                      +                                  V                  th                                -                                  V                  th                                            )                        2                                                  =                  I          sig                    
When Vgs obtained from Expression 3 is substituted into Vgs in Expression 4 and Expression 4 is then rearranged, finally, the terms of the mobility μ and the threshold voltage Vth are canceled so that a relationship of Ids=Isig is obtained. Consequently, even when the mobility μ and threshold voltage Vth of the drive transistor Trd disperse among the individual pixels, the dispersion in the mobility μ and threshold voltage Vth of the drive transistor Trd is canceled by performing the above-mentioned signal current writing operation, and thus the uniformity of the picture can be maintained.
The conventional pixel circuit shown in FIG. 23 has such an advantage that the drive current Ids equal to the signal current Isig can be supplied to the electroluminescence element EL irrespective of the dispersion in mobility μ and threshold voltage Vth of the drive transistor Trd. The current driver 3 can change the luminance of the electroluminescence element EL from the black level up to the white level through the intermediate gray level by gradation-controlling the signal current Isig. When the luminance of the electroluminescence element EL is at the black level, the signal current Isig becomes weak so that its magnitude approaches zero, while when the luminance of the electroluminescence element EL is at the white level, the signal current Isig becomes a large current. However, the parasitic capacity of the signal line SL takes a relatively large value, i.e., several tens of pF. As a result, there is encountered such a problem that with the conventional structure shown in FIG. 23, the weak signal current Isig when the luminance of the electroluminescence element EL is at the black level cannot be sufficiently written within one horizontal image period (1H) allocated to the sampling operation.
FIG. 24 is a diagram schematically representing this problem. A case is shown where a pixel array 1 constitutes a picture, and a white window is displayed against a black background on the picture area. A gray portion appears under the while window. Essentially, this gray portion belongs to the background and thus must be black. However, with the conventional pixel circuit structure shown in FIG. 23, the signal current corresponding to the block level cannot be written to any of the pixels located under the white window. Hence, the black embossing, the longitudinal cross-talk or the like as shown in FIG. 24 is generated. This becomes a problem to be solved.